A spark plug is mounted to, for example, an internal combustion engine (an engine) for igniting an air-fuel mixture contained in a combustion chamber. Generally, a spark plug includes an insulator having an axial bore; a center electrode inserted into a forward end portion of the axial bore; a metallic shell provided externally of the outer circumference of the insulator; and a ground electrode which is resistance-welded to a forward end portion of the metallic shell and forms a spark discharge gap in cooperation with the center electrode. Also, in order to improve corrosion resistance, the metallic shell to which the ground electrode is welded may be subjected to plating, such as zinc plating.
Meanwhile, since the ground electrode is disposed in such a manner as to project toward the center of a combustion chamber, the temperature of the ground electrode becomes very high during use. Accordingly, the ground electrode is apt to suffer a problem that crystals of a metal material used to form the ground electrode coarsen (so-called grain growth), potentially resulting in internal corrosion and a deterioration in durability.
In order to restrict grain growth, there has been proposed a technique for forming the ground electrode from a metal material in which precipitates of an oxide, a nitride, or an intermetallic compound of Y, Zr, etc., are precipitated in grain boundaries (refer to, for example, Patent Documents 1 and 2). According to the technique, in the process of coarsening of crystal grains at high temperature, the precipitates hinder the growth of crystal grains; as a result, grain growth is restrained, whereby internal corrosion or the like can be effectively prevented.